Well, they're plants which to me symbolize life, and necromancers are all about death. I dunno. :P

Well, death is often a process of rebirth in nature. So perhaps it's not too off for a Sylvari to train in death magic. The real contradiction I see is that Necromancy inherently deals with the undead- which would be an abomination to the natural world.

For example a Sylvari necromancer would seem weird to me. But a charr engineer fits the best in my opinion.

"Death as well is still not fully understood, but it is viewed as something to be embraced and wholly understood - this creates an acceptance and fascination of death which the other races can find disturbing." Taken from GW2 wiki on the Sylvari page. This is probably why some sylvari become necros. As for me, I hate the idea of asura warriors. Tiny and feeble creatures shouldn't be able to ground pound the earth.